A nicely-coloured Baillon’s wrasse (Symphodus bailloni) from Southampton Water last week (Jan. 16th). We’ve not seen an adult in breeding colours in this location before; note the pink fins and mouth and black spot towards the rear of the dorsal fin. It does have a light spot on the base of the tail fin, and confusion can occur with corkwing wrasse because of this.

Around the head, note the orange blobs on a blue background, the dark-blue arc at the base of the pectoral fin, and Prussian blue colour behind the pectoral fin.

Sand goby, Pomatoschistus minutus. The black/blue spot on the first dorsal fin is at the upper edge of the fin. The common goby has a similar spot, but it is positioned lower on the fin membrane, close to the body.

31st January 2019
A large adult male sand goby, Pomatoschistus minutus, from Southampton Water. The images show the bright blue colouration of the anal fin which is characteristic of the males' breeding colours, and forms part of their mating ritual. As Blom, Wilson, Kvarnemo, Amorim and Svensson (2022) explain in "Male acoustic display in the sand goby – Essential cue in female choice, but unaffected by supplemental feeding":
"Males develop a nuptial colouration with black pigmentation on pelvic, anal and tail fins. The anal fin has a clear blue colouration with a black trim, and there is a eye-spot on the first dorsal fin. This colouration is lacking in the cryptic female. Males build a nest by covering bivalve shells with sand and compete for mating opportunities with females, which in turn are the choosier sex. Male courtship behaviour normally starts with a visual display in which the male approaches the female with erect ornamented fins. Typically, the male swims back to the nest in a conspicuous manner, considered a lead display (‘lead swim’), and the female may choose to follow the courting male to his nest . The male might produce a series of sounds both when the female is outside the nest and when she has entered it".
(full text available here).

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Adult sprats (Sprattus sprattus) and transparent goby (Aphia minuta) bear no resemblance at all. Juvenile sprats and gobies, on the other hand, are harder to tell apart in the field, since both are to some degree transparent and more or less the same size. Sprat have a single dorsal fin, while gobies have two, though this difference isn't always easy to spot without a lens. The most obvious difference is shown in the image below; the sprat has a long silvery gut, while in the transparent goby the only visible internal structure is the round, bubble-like swim bladder. The difference isn't always this visible.
Unfortunately, at that size, sprats can also easily be mistaken for juvenile sand smelts (Atherina presbyter or A. boyeri); there, the most reliable indicator is to count the dorsal fins - smelts have two.

All content © Pisces Conservation Ltd, unless otherwise indicated.

Most people who encounter a lesser weever (Echiichthys vipera) keep as far away from it as possible; a pity (from a scientific point of view, at least), since they are occasionally host to an intriguing and macabre parasite. Ceratothoa steindachneri, known as the tongue-biter, is an isopod (an order of crustaceans which also includes woodlice) with a strange life-cycle. All individuals begin life as males, and find their home inside the mouth of their host fish. They attach themselves to the fish's tongue, and cut off the blood supply, so that the tongue withers away and falls off; the isopod remains in place, and from then on must fulfil at least some of the tongue's functions.
The parasite then changes sex, becoming female, and waits for another male to enter the fish's mouth, so that they can reproduce.
Until 2021, Ceratothoa steindachneri had only been observed in weevers on the Cornish coast. However, in April that year we caught 3 weevers in the Medway estuary in Kent, each of which had a pair of tongue-biters in their mouths.
It seems likely that they are much more commonly-occurring around our coast - however, until more people start looking in weevers' mouths, we might never find out.
More on Ceratothoa steindachneri here and here.

All content © Pisces Conservation Ltd, unless otherwise indicated.